Simply put, does the drawing left of the red line (Supply) match the right side (Demand) in terms of air flow and pressure? Does it continually match? Likely it does not as demand is in constant fluctuation whether from changes in use, shift change or downtime.

Within a compressed air system, the system dynamics (changes in demand over time) are especially important. Using controls, storage, and demand management to effectively design a system that meets peak requirements but also operates efficiently at part-load is key to a high performance compressed air system.

In many systems, compressor controls are not coordinated to meet the demand requirements, which can result in compressors operating in conflict with each other, short-cycling, or blowing off are all signs of inefficient system operation.

As in most relationships, the key is communication. Communication between the compressors so that each (or a central controller) knows the status of all the other units. Communication is also key between the compressors (or central controller) and the demand side of the system. The compressors perform based on information from a pressure sensor located downstream of the compressor discharge. But where is this sensor located?

A common mistake is the pressure sensor is located close to the compressor discharge. Perhaps even before the dryer. If this is the case in our system drawing above, you can see that the compressor is performing based on pressure data before the air enters the dryer or the subsequent filter, both of which have pressure drop as air passes through. This drop can range from 5 PSIG up to 12 PSIG or higher depending on how well the system was designed as well as the condition of the filter element. A dirty element near the end of it’s life will have substantially more pressure differential than a clean filter element.

If the compressors are discharging at 100 PSIG and there is a 10 pound drop through the dryer and filter, then the plant is only receiving 90 PSIG and this doesn’t include the pressure drop throughout the plant piping system. When a cyclical event occurs, such as several blow guns operating at the same time, the pressure drop has to travel back through the entire system before the compressors see the event and can respond to correct the drop in pressure by increasing their output.

All factors must be considered when designing or updating a compressed air system. Your best course of action is to consult with a compressed air professional to assist in your design or upgrades prior to writing any equipment specifications.

Conservation Strategies for Compressed Air Systems

Identify easy to implement energy conservation opportunities in your compressed air system by conducting a walk-through assessment. Simple conservation opportunities can result in savings up to 25% of the current cost to run the compressed air system.

Leaks

Routinely check your system for leaks. A distribution system under 100 pounds-per-square-inch gauged (psig) of pressure, running 40 hours per week, with the equivalent of a quarter-inch diameter leak will lose compressed air at a rate of over 100 cfm costing over $2,800 per year. In noisy environments an ultrasonic detector may be needed to locate leaks.

Compressor pressure

The compressor must produce air at a pressure high enough to overcome pressure losses in the supply system and still meet the minimum operating pressure of the end use equipment. Pressure loss in a properly designed system will be less than 10% of the compressor’s discharge pressure found on a gage on the outlet of the compressor. If pressure loss is greater than 10%, evaluate your distribution system and identify areas causing excessive pressure drops. Every two PSI decrease in compressor pressure will reduce your operating costs 1.5%.

Identify artificial demands

Artificial demand is created when an end use is supplied air pressure higher than required for the application. If an application requires 50 psi but is supplied 90 psi, excess compressed air is used. Use pressure regulators at the end use to minimize artificial demand.

Inappropriate use of compressed air

Look for inappropriate uses of compressed air at your facility. Instead of using compressed air, use air conditioning or fans to cool electrical cabinets; use blowers to agitate, aspirate, cool, mix, and inflate packaging; and use low-pressure air for blow guns and air lances. Disconnect the compressed air source from unused equipment.

Heat recovery

As much as 80 to 90% of the electrical energy used by an air compressor is converted to heat. A properly designed heat recovery unit can recover 50 to 90% of this heat for heating air or water. Approximately 50,000 British thermal units (BTUs) per hour is available per 100 cfm of compressor capacity when running at full load. For example, consider a 100 hp compressor that generates 350 cfm at full load for 75% of the year. If 50% of heat loss is recovered to heat process water, the savings, at $0.50 per therm, would be about $4,100 per year in natural gas.

Inlet air filters

Maintain inlet air filters to prevent dirt from causing pressure drops by restricting the flow of air to the compressor. Retrofit the compressor with large-area air intake filters to help reduce pressure drop.

Compressor size

If your compressor is oversized add a smaller compressor and sequence-controls to make its operation more efficient when partially loaded. Sequence-controls can regulate a number of compressors to match compressed air needs, as they vary throughout the day.

Air receiver/surge tank

If your compressed air system does not have an air receiver tank, add one to buffer short-term demand changes and reduce on/off cycling of the compressor. The tank is sized to the power of the compressor. For example, a 50 hp air compressor needs approximately a 50-gallon air receiver tank.

Cooler intake air

When ingesting cooler air, which is more dense, compressors use less energy to produce the required pressure. For example, if 90 degree F intake air is tempered with cooler air from another source to 70 degree F, the 20 degree F temperature drop will lower operating costs by almost 3.8%.

V-belts

Routinely check the compressor’s v-belts for proper tightness. Loose belts slip more frequently which reduces compressor efficiency.

An air audit performed by a professional auditing company can find tremendous savings in your compressed air system. Consider an audit as an investment for your companies future.

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